* Risk of rebellion in conservative ranks

* Bild says up to 120 CDU/CSU members may vote against bailout

* Schaeuble says scope for Greek debt relief not very big (Adds Greens reaction, CDU comment, detail on Merkel trips)

By Noah Barkin

BERLIN, Aug 15 (Reuters) - In a major test of her authority, Chancellor Angela Merkel will ask sceptical German lawmakers to back an 86 billion euro ($95.5 billion) bailout for Greece on Wednesday despite uncertainty over whether the IMF will play a role in the rescue.

Parliamentary approval is not in doubt because the Social Democrats (SPD) and Greens are expected to back the deal. But the vote could expose a deep divide among Merkel's conservatives, damaging the German leader and her close ally Volker Kauder, the head of her bloc in parliament.

Kauder, who incensed fellow lawmakers last week with threats of retaliation if they rebelled and voted against a bailout, has described the involvement of the International Monetary Fund as a "condition" for the support of his party.

However under the bailout approved by euro zone finance ministers at a meeting in Brussels late on Friday, it is unclear whether the IMF will end up playing a role.

IMF Managing Director Christine Lagarde told the ministers by telephone that she could not commit until her board reviewed the situation in the autumn. She renewed a call for "significant" debt relief for Greece, a demand Merkel's government has repeatedly pushed back against.

German Finance Minister Wolfgang Schaeuble reiterated his opposition to an outright writedown of the face value of Greek debt in an interview with Deutsche Welle published on Saturday. He said the scope for milder forms of debt relief, like extending debt maturities, was "not very big".

The IMF took part in the first two rescues for Greece, which totalled 240 billion euros, and Berlin is keen to keep it on board because of the Washington-based institution's reputation for rigour.

RISK OF BIG REBELLION

Last month, a record 65 lawmakers from Merkel's conservative camp broke ranks and refused to back negotiations on the bailout.

Far more could rebel in Wednesday's vote, with top-selling German daily Bild estimating that up to 120 members of her Christian Democratic Union (CDU) and its Bavarian sister party, the Christian Social Union (CSU), may refuse to back the government.

The Bundestag debate on Greece is scheduled to start at 9 a.m. (0700 GMT) on Wednesday, disrupting Merkel's travel plans.

She had been scheduled to leave on a government trip to Brazil around that time. Her spokesman Steffen Seibert said the departure would be delayed by several hours.